1. In 5.8, there may be a trade-off between which group has the harps and
which as the bowls of incense: I suggest the 24 elders have the harps and
the creatures the bowls, forming a chiasmus within this sentence. Harps
require 2 hands, and large bowls do also. This arrangement suggests that it
is the elders who are singing in 5.9, not the beasts.

2. Even if the beasts are singing along, from general linguistic evidence I
would argue that pronouns are flexible enough to be used with some
imprecision. That is, pronouns never cut up the space of all possible
participants with as many divisions as various circumstances call for. In
this case, there is no pronoun for we in this group but not those over
there. Further, those over there could potentially join in the chorus,
sympathizing with the sentiments of the elders without compromising their
integrity.

For example, I recall singing Ps 115.1-3 set to music in my church years
ago; these verses are replete with "us" and "our", but we had no problem
singing the words, recognizing their historical application to Israel, and
identifying with their sentiments of praise to God.